A substance injected under the skin to enhance animal growth is known as a:

A substance injected under the skin to enhance animal growth is identified as an implant. Implants are commonly used in livestock to deliver hormones or other growth-promoting substances directly into the body, promoting improved growth rates and feed efficiency. The term "implant" specifically refers to this method of growth enhancement, which is often used in the cattle industry, among other livestock species.

Vaccine, while also administered through injection, serves a different purpose by stimulating the immune system to prevent disease rather than enhancing growth. Supplements are typically added to feed or provided separately to improve nutrition but are not necessarily injected. Antibiotics are used to prevent or treat infections but do not specifically enhance growth in the same way as growth implants do. Therefore, the choice of "implant" accurately reflects the specific use of a substance for growth promotion through injection under the skin.